I had that problem with my Big Game. I was sending a Sim from Sunlit Tides to Legacy Island III which is no longer working properly due to being played for too long and having had too many sims in it. It has memory issues. I had my graphics up to maximum (or close to) and the sims just couldn't arrive. After a brief panic I experimented and turned down my graphics to minimums all over and had another go. I sent my sim to Legacy Island III and this time he arrived safely. I then used him to grab a few households to take them to a more playable world.
I had the little panic as I still have a heap of Sims in Legacy Island III that I want to keep in my Big Game and for a while I thought I wouldn't be able to retrieve them without going back to earlier saves and porting them in and then fixing up relationships, even if it would have been possible to fix the relationships given the number of sims involved.
Now, whenever I send a Sim to Legacy Island III I make sure my graphics are down at minimums. So for me it was a memory issue.
I don't know if there are other issues that cause this problem.
If you can't get to Isla Paradiso, and I'm having that problem with my Isla Paradiso Bunch videos now, you can send them to another world with dive lots in it.
I recently did a video people are finding helpful to build dive lots in other worlds. It is all done step by step so if you follow each step you will end up with a working dive lot in most worlds that have sufficiently deep salt water. The world I used in my video was Sunlit Tides.